Typically, when a business enterprise or the like constructs a private network, a method of construction using a private line, a method of construction by means of an IP-VPN (Virtual Private Network) using IP (Internet Protocol), or a method of construction by means of a wide range LAN service using a VLAN (Virtual LAN) is employed. Among these methods, construction methods using a wide range LAN service are becoming more widespread due to the low cost and easy management of a wide range LAN service in comparison with a private line or IP-VPN.
Among wide range LAN services, VLAN over VLAN technology, which allows a user to perform VLAN allocation freely with no restrictions from the carrier, is gaining attention.
In VLAN over VLAN technology, a VLAN frame received from a user is transferred over the carrier network transparently, and hence the ingress node of the carrier network only attaches (stacks) VLAN information (a VLAN tag) which is valid within the carrier network to a frame received from a user network. Each node in the carrier network transfers the frame using the VLAN tag and a destination MAC address. The egress node of the carrier network deletes the VLAN tag stacked onto the frame and transfers the frame to the user network. In so doing, the user network is provided with free VLAN allocation.
Meanwhile, to prevent the same frame from being transferred repeatedly between nodes in a loop form, the nodes providing the wide range LAN service construct a network having a tree structure with no loops using a spanning tree protocol (STP) conforming to IEEE802.1D.
In STP, the lower bridge of the tree structure uses the information of a BPDU (Bridge Protocol Data Unit), which is a control frame received from the upper bridge, to recalculate the spanning tree. The BPDU is not stacked with a VLAN tag, and therefore a BPDU transmitted from the user network (LAN or the like) of a user A may also be transmitted to the user network of another user B. For example, in FIG. 27, a BPDU transmitted from a user network 501 of a user A may be transmitted not only to a user network 502 of the same user A, but also to a user network 601 and/or a user network 602 of another user B.
Moreover, when bridges are added to and deleted from user networks such that the configuration of the user network changes, or when the root bridge (a bridge 510 in FIG. 27, for example) is damaged, the next root bridge may become a node of the carrier network or a bridge on the user network of the user B. Such changes in the configuration of the network of a certain user may affect the user network of another user.
To solve this problem, a method of constructing spanning trees independently in a user network and a carrier network has been considered. In FIG. 27, for example, a spanning tree is constructed between the respective user networks 501, 502 of the user A, a spanning tree is constructed between the respective user networks 601, 602 of the user B, and a spanning tree is constructed for a carrier network 700.
In this method, a spanning tree is constructed by discarding a BPDU entering the carrier network from the user network at an ingress node 701.
As shown in FIG. 28, however, with this method, when a dual homing configuration connecting a plurality of lines is constructed between the user network and carrier network as a damage prevention measure, the STP becomes incomplete between the user networks spanning the carrier network such that the occurrence of loops is not detected. As a result, transfer frames are caught in an infinite loop.
To solve this problem, a method may be employed in which an STP is not used during user network construction, and instead a maintenance person performs setting manually in consideration of the topology, ensuring that no loops occur in the user network. With this method, however, the configuration of the networks spanning the carrier must be taken into consideration, and transfer routes must be set for all of the MAC addresses used in the user networks, which is unrealistic.
Hence when a dual homing configuration is constructed, a network without loops is provided by disposing a router on the edge side of the user network.
With this method, however, the use of a router causes an increase in the network construction cost. Moreover, this method cannot be called a wide range LAN service, which provides a VLAN using a reasonably-priced network device such as a bridge, and hence in essence, a wide range LAN service cannot be provided.
Meanwhile, in optical networks and the like, a “Method for routing traffic between network components to protect network against fault”, disclosed in Japanese Unexamined Patent Application Publication 2002-57713, has been proposed as a dual home approach providing protection against network faults.